Generally only a portion of the petroleum in a reservoir can be produced by primary recovery methods, e.g., methods that rely on the energy in the formation for production. Secondary recovery methods such as water-flooding can result in further production, but large proportions of the original petroleum can remain in the reservoir even after application of primary and secondary methods. Enhanced oil recovery methods can substantially increase production beyond the yields obtained using primary and secondary recovery.
Enhanced oil recovery methods can include the injection of a viscosified aqueous solution through an injection wellbore and into the producing zone to sweep recoverable petroleum toward a production wellbore. However, high temperatures downhole or high concentrations of salt can cause viscosifiers in the aqueous solution to lose effectiveness, decreasing the viscosity of the aqueous solution. The lower viscosity of the aqueous solution can cause the interface between the aqueous solution and the remaining petroleum to experience “fingering,” in which the aqueous solution passes by petroleum material, resulting in a less effective sweeping action and a bypassing of recoverable petroleum.